Abstract:

A method for coating a polymeric or composite component surface with a
wear and erosion resistance metal layer includes the step of cold
gas-dynamic spraying a powder mixture onto the polymeric or composite
component surface to form the wear and erosion resistance metal layer.
The mixture may include at least one metal powder and at least one hard
particle powder.

Claims:

1. A method for forming a dense and continuous metal coating on a
polymeric component surface of a polymeric component, the method
comprising:cold gas dynamic spraying a metal powder having a first
average particle diameter directly onto the polymeric component surface
at a first particle velocity sufficient to cause the metal powder to
micropenetrate the polymeric component surface and also form the dense
and continuous metal coating thereon to protect the polymeric component.

2. The method of claim 1, further comprising:cold gas dynamic spraying a
metal powder having a second average particle size that is smaller than
the first average particle size as part of the dense and continuous metal
coating, after spraying the metal powder having the first average
particle size.

3. The method of claim 2, wherein the metal powder having the second
average particle size is sprayed at a second particle velocity that is
higher than the first particle velocity to ensure deformation and bonding
of the metal powder.

4. The method according to claim 1, wherein the dense and continuous metal
coating formed directly on the polymeric component substrate is a heat
shielding metal layer having a thickness of at least 5.1 mils.

5. The method according to claim 1, wherein the dense and continuous metal
coating formed directly on the polymeric component substrate has a
thickness of less than 2.0 mils.

6. The method according to claim 1, wherein the polymeric component
surface is selected from the group consisting of a polycarbonate,
polytetrafluoroethylene, nylon, polyoxymethylene, polysulfone,
polyphenylene, and polyamide.

7. The method according to claim 1, wherein the metal powder comprises at
least one metal selected from the group consisting of aluminum, copper,
silver, zinc, magnesium, iron, brass, bronze, nickel, and titanium.

8. A method for coating a polymeric component surface of a polymeric
component with a heat shielding metal layer, the method comprising:cold
gas-dynamic spraying a first metal powder onto the polymeric component
surface to form a bonding layer;cold gas-dynamic spraying a second metal
powder onto the bonding layer to form the heat shielding metal layer at a
thickness of at least 5.1 mils, the heat shielding metal layer protecting
the polymeric component from external high temperatures.

9. The method according to claim 8, wherein the polymeric component
surface comprises at least one polymer selected from the group consisting
of a polycarbonate, polytetrafluoroethylene, nylon, polyoxymethylene,
polysulfone, polyphenylene, and polyamide.

10. The method according to claim 8, wherein at least the first metal
powder comprises at least one soft metal selected from the group
consisting of aluminum, copper, silver, zinc.

11. The method according to claim 8, wherein the second metal powder
comprises at least one intermediate hardness metal selected from the
group consisting of magnesium, iron, brass, bronze, nickel, titanium,
chromium, steel, and MCrAlY alloys wherein M is a metal.

12. A method for coating a polymeric component surface of a polymeric
component with a dense and uniform wear and erosion resistance metal
layer, the method comprising:cold gas-dynamic spraying a powder mixture
onto the polymeric component surface to form the dense and uniform wear
and erosion resistance metal layer that protects the polymeric component,
the mixture comprising at least one metal powder and at least one hard
particle powder.

13. The method according to claim 12, wherein the polymeric component
surface comprises at least one polymer selected from the group consisting
of polycarbonate, polytetrafluoroethylene, nylon, polyoxymethylene,
polysulfone, polyphenylene, and polyamide.

14. The method according to claim 12, wherein the metal powder comprises
at least one soft metal selected from the group consisting of aluminum,
copper, silver, zinc, magnesium, iron, brass, bronze, nickel, and
titanium.

16. A method for forming a dense and continuous metal coating on a
component surface formed from a non-metallic composite material, the
method comprising:cold gas dynamic spraying a metal powder directly onto
the component surface to form the continuous metal coating thereon.

17. The method according to claim 16, wherein the non-metallic composite
material is a fiber-reinforced composite material.

19. The method according to claim 16, wherein the metal powder comprises
at least one metal selected from the group consisting of aluminum,
copper, silver, zinc, magnesium, iron, brass, bronze, nickel, titanium,
chromium, steel, and MCrAlY alloys wherein M is a metal.

20. The method according to claim 16, wherein the metal powder further
comprises at least one hard particle powder mixed therein that is wear
and erosion resistant.

Description:

TECHNICAL FIELD

[0001]The present invention relates to methods for applying dense and
well-bonded metal coatings onto polymeric articles such as airframe
components and, more particularly, to methods for coating at temperatures
below the melting points of the materials that form the coatings and
components.

BACKGROUND

[0002]Cold gas-dynamic spraying (hereinafter "cold spraying") is a
technique that is sometimes employed to form coatings of various
materials on a substrate. In general, a cold spraying system uses a
pressurized carrier gas to accelerate particles through a supersonic
nozzle and toward a targeted surface. The cold spraying process is
referred to as a cold process because the particles are mixed and sprayed
at a temperature that is well below their melting point, and the
particles are near ambient temperature when they impact with the targeted
surface. Converted kinetic energy, rather than a high particle
temperature, causes the particles to plastically deform, which in turn
causes the particles to form a bond with the targeted surface. Bonding to
the component surface occurs as a solid state process with insufficient
thermal energy to transition the solid powders to molten droplets. Cold
spraying techniques can therefore produce a thermal or wear-resistant
coating that strengthens and protects the component using a variety of
materials that may not be easily applied using techniques that expose the
materials and coatings to high temperatures.

[0003]A variety of different systems and implementations can be used to
perform a cold spraying process. For example, U.S. Pat. No. 5,302,414,
entitled "Gas-Dynamic Spraying Method for Applying a Coating" describes
an apparatus designed to accelerate to supersonic speed materials having
a particle size of between 5 to about 50 microns. The particles are
sprayed from a nozzle at a velocity ranging between 300 and 1200 m/s.
Heat is applied to the carrier gas to between about 300 and about
400° C., but expansion in the nozzle causes the spraying material
to cool. The spraying material therefore returns to near ambient
temperature by the time it reaches the targeted substrate surface.

[0004]When coating metal substrates, the sprayed particles impinge on the
targeted substrate surface and the impact breaks up any oxide films on
the particle and substrate surfaces as the particles bond to the
substrate. Thus, cold spraying techniques prevent unwanted oxidation of
the substrate or powder, and thereby produce a cleaner coating than many
other processes. Such techniques also enable the formation of
non-equilibrium coatings. More specifically, since the sprayed materials
are not heated or otherwise caused to react with each other or with the
substrate, coatings can be produced that are not producible using other
techniques.

[0005]In contrast to cold spraying, thermal spraying processes include
heating methods to bring at least some of the spray material to a melting
point prior to impacting the sprayed surface, thereby producing a strong
and uniform coating. Some thermal spraying processes also utilize plasma
to ionize the sprayed materials or to assist in changing the sprayed
materials from solid phase to liquid or gas phase. Melted spraying
particles produce liquid splats that land on a targeted substrate surface
and bond thereto. Some thermal spraying techniques only supply sufficient
heat to melt a fraction of the spraying material particles, and
consequently only cause surface melting to occur.

[0006]Thermal spraying is not a viable method for applying coatings of
alloys having relatively high melting temperatures to many substrates
since the high temperature liquid or particles may react with or disrupt
the substrate surface and perhaps lower its strength. For example,
plastic and other polymeric materials typically have relatively low
melting temperatures when compared to metals, and would consequently melt
and/or burn upon impact with molten metals. Cold spraying is sometimes a
preferred spraying method for various substrates because it enables the
sprayed materials to bond with such substrates at a relatively low
temperature. The coating materials that are sprayed using the cold
gas-dynamic spraying process typically only incur a net gain of about
100° C. with respect to the ambient temperature. Plastic
deformation facilitates bonding of sprayed particles to the substrate.
Further, since the sprayed particles are kept well below their melting
temperatures, they are not very susceptible to oxidation or other
reactions.

[0007]The family of polymers covers a wide range of materials, although
most polymers are generally lightweight. Polymers are often easily
formable by various processes, and consequently may be used to
manufacture complex shapes. Some polymer materials have somewhat low
strength, although many have strengths comparable to aluminum, and others
such as carbon composites are very strong and rigid. As previously
mentioned, however, polymers commonly have low melting temperatures and
consequently may deform, burn, or be otherwise damaged by exposure to
temperatures far below the melting points of many metals. Further,
although many polymers are strong and rigid, many are not very wear
resistant. These limitations reduce the usefulness of polymers for
applications in some high technology applications such as in the
aerospace field. Although polymers may be useful structural materials for
some airframe components, they have limited utility in areas that are
close to an engine, a heat exchanger, or an auxiliary power unit.
Further, polymers have limited utility as materials for noise suppression
or vibration damping components because there is often a likelihood
exposure to erosion-promoting elements and/or to damaging heat.

[0008]The composite family also covers a wide range of materials. Some
such as carbon/carbon and silicon/silicon carbide are fiber reinforced
and have exceptionally high strengths. Composites made from Spectra®
fiber fabricated by Honeywell International, Inc. also have expansive
utility as structural materials in some high technology applications.
However, utility is somewhat limited for composites since many have poor
erosion or temperature capabilities.

[0009]Hence, there is a need for low cost methods of protecting substrates
made from polymeric materials in order to enhance the wear-resistance,
and usefulness of polymeric components in high temperature environments.
There is also a need for a method that is capable of efficiently and
cost-effectively producing a wear and temperature-resistant coating for
polymeric components. There is also a need for a spraying method by which
such coatings may be uniformly and thoroughly applied at temperatures
well below their melting points.

BRIEF SUMMARY

[0010]The present invention provides a method for forming a continuous
metal coating on a polymeric component surface with a metallic coating.
The method includes the step of cold gas dynamic spraying a metal powder
having a first average particle diameter directly onto the polymeric
component surface at a first particle velocity sufficient to cause the
metal powder to micropenetrate the polymeric component surface and also
form the continuous metal coating thereon.

[0011]The present invention also provides a method for coating a polymeric
component surface with a heat shielding metal layer. The method includes
the steps of cold gas-dynamic spraying a first metal powder onto the
polymeric component surface to form a bonding layer, and then cold
gas-dynamic spraying a second metal powder onto the bonding layer to form
the heat shielding metal layer at a thickness of at least 5.1 mils.

[0012]The present invention also provides a method for coating a polymeric
component surface with a wear and erosion resistance metal layer. The
method includes the step of cold gas-dynamic spraying a powder mixture
onto the polymeric component surface to form the wear and erosion
resistance metal layer, the mixture comprising at least one metal powder
and at least one hard particle powder.

[0013]The present invention also provides a method for forming a
continuous metal coating on a component surface formed from a
non-metallic composite material. The method includes the step of cold gas
dynamic spraying a metal powder directly onto the component surface to
form the continuous metal coating thereon.

[0014]Other independent features and advantages of the preferred methods
will become apparent from the following detailed description, taken in
conjunction with the accompanying drawing which illustrates, by way of
example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING

[0015]FIG. 1 is a schematic view of a cold spraying apparatus;

[0016]FIG. 2 is a block diagram depicting an exemplary method of cold
spraying a metal coating onto a polymeric substrate;

[0017]FIG. 3A is a 30× microscopic image of a cold sprayed
aluminum/aluminum oxide coating formed on a polycarbonate substrate;

[0018]FIG. 3B is a 200× microscopic image of the coating depicted in
FIG. 3A; and

[0019]FIG. 4 is a 300× microscopic image of a partial coating of
aluminum on a polycarbonate substrate.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0020]The following detailed description of the invention is merely
exemplary in nature and is not intended to limit the invention or the
application and uses of the invention. Furthermore, there is no intention
to be bound by any theory presented in the preceding background of the
invention or the following detailed description of the invention.

[0021]Turning now to FIG. 1, an exemplary cold spraying system 100 is
illustrated diagrammatically. The system 100 is illustrated as a general
scheme, and additional features and components can be implemented into
the system 100 as necessary. The main components of the cold spraying
system 100 include a powder feeder 22 for providing powder materials, a
carrier gas supply 24, a mixing chamber 26 and a convergent-divergent
nozzle 28.

[0022]In general, the system 100 transports the metal powder mixtures with
a suitable pressurized gas to the mixing chamber 26. The particles are
accelerated by the pressurized carrier gas through the specially designed
supersonic nozzle 28. Exemplary carrier gases include air, helium and
nitrogen. When the powder particles are accelerated toward the nozzle 28,
the carrier gas is typically heated to about 300 to 400° C. The
nozzle 28 directs the accelerated powder particles toward a targeted
surface 10 to form a dense and uniform coating. Due to expansion in the
nozzle, the powder particles are close to ambient temperature when they
impact with the targeted surface 10. If the particles reach a critical
velocity, which is specific to each type of powder, the impact will cause
any oxide films on the particles to break up. Further, the kinetic energy
associated with the impact causes plastic deformation of the particles,
and further causes the particles to bond to the targeted surface 10.

[0023]Because the cold spraying system 100 is useful for depositing
coatings at temperatures far below the sprayed material melting point,
cold spraying may be a uniquely capable process for forming coatings on
polymeric and components and composite materials as heat shields and/or
environmental barriers that promote wear and erosion resistance.
Thermoplastic materials are an exemplary set of polymers on which metal
coatings may be formed by cold spraying. Polycarbonates are just one type
of suitable thermoplastic material. Exemplary polycarbonate substrates
include any in the family of Lexan® polycarbonate thermoplastic
resins. Such resins are amorphous engineering thermoplastics with high
mechanical, optical, electrical and thermal properties. They may include
a variety of additives, including UV stabilizers, mold release agents,
flame retardants, and glass materials as structural reinforcement
additives. Other suitable polymers include polytetrafluoroethylene
(teflon®), nylon, polyoxymethylene (acetal), polysulfone,
polyphenylene, and polyamide. Fiber-reinforced composites are an
exemplary set of non-metallic composite materials on which metal coatings
may be formed by cold spraying. Exemplary fiber-reinforced composite
materials include Kevlar®, and Spectra® fibers. Fibers of
Kevlar® include long molecular chains produced from poly-paraphenylene
terephthalamide. Spectra® fibers include strands of polyethylene with
tetrahedral-bonded carbon atoms that provide much higher strength and
melting temperatures to the composition than standard polyethylene. Other
exemplary composites include carbon fiber-reinforced composites,
silicon/silicon carbide-reinforced composites, polytetrafluoroethylene
fiber-reinforced composites. Various other particulate, discontinuous
fiber, and continuous fiber composites may also be suitable substrates.

[0024]Metal coatings may be cold sprayed onto a polymeric component with
varying thicknesses depending on the coating's intended purpose. Both
thin and thick coatings may be adequately bonded to the component.
Further, since bonding may be more effective between particular polymers
and metals than others, the metal coatings may comprise a thin layer of
an effective bonding metal formed directly on the polymer substrate, and
at least one thicker metal layer formed on the thin bonding metal layer.
The at least one thicker layer, and particularly the outermost thicker
outer layer, has desired wear and erosion resistant properties to protect
the polymer substrate.

[0025]One exemplary cold sprayed metal coating is formed directly on a
polymeric component as a thick single layer. The thick metal coating
functions as a heat shield. More particularly, the thick metal coating is
capable of both reflecting and conducting external heat to protect the
polymeric component from external high temperatures. An additional
advantage is that the coating acts as an oxygen barrier that prevents the
polymer from burning. An effective heat shield coating is cold sprayed is
at least 5.1 mils (at least 130 micrometers). The thick metal coating is
preferably formed directly on the polymeric component, although if
necessary a single thin bonding layer may be formed between the thick
metal coating and the polymeric component to promote mechanical and/or
chemical bonding. According to other embodiments, the overall metal
coating formed directly on the polymeric component is less than 2.0 mils
(less than about 50 microns). For example, an oxidation (i.e. burning)
and/or corrosion barrier may be sufficient at a thickness of less than
1.0 mil (less than about 25 microns).

[0026]There is a wide range of achievable particle velocities using a cold
spray system. When coating a polymeric component surface with a metal,
the carrier gas pressure may be adjusted as necessary to ensure that the
polymer substrate is not eroded or otherwise damaged upon impact with the
sprayed solid metal particles. For example, when spraying hard metals the
carrier gas pressure may be reduced initially to a relatively low
pressure until the metal forms a thin coating on the substrate. Once the
thin coating is formed, the carrier gas pressure may be raised to a
relatively high pressure to form the remainder of the metal coating on
the polymer substrate.

[0027]In addition, soft metals may be selected over harder metals
depending on the specific polymer substrate. For example, metals such as
aluminum, copper, silver, zinc, and other relatively soft metals, and
combinations of such metals, may be readily cold-sprayed onto most
polymeric substrates at a velocity at which plastic deformation and
bonding will occur without damage to the substrates. For other
intermediate-hardness metals such as magnesium, iron, brass, bronze,
nickel, titanium, and other medium-hardness metals, and combinations of
such metals, it may be beneficial to reduce the carrier gas pressure so
the metal only partially deforms upon impact with the polymer substrate.
Alternatively, it may be beneficial to form a bonding layer of a soft
metal and then to deposit a medium-hardness metal to avoid damage to the
substrate. For hard metals such as chromium, steel, MCrAlY alloys, and
various hard superalloys, a bonding layer may be necessary depending on
the hardness of the polymer substrate.

[0029]Turning now to FIG. 2, a block diagram depicts an exemplary method
for coating a polymeric substrate with a metal. Starting with step 30,
one or more metals are selected for spraying on a polymer substrate. The
substrate may be a surface of any type of component. As previously
mentioned, the coating method benefits polymeric components that are may
be susceptible to heat-related damage and/or erosion due to high
temperatures and environmental hazards. Exemplary polymeric components
that may benefit from cold sprayed metal coatings include components
included in aerospace and other high technology applications, although
there are countless other applications for which the present invention
may be beneficial. As another example, a thick coating may be useful to
enhance rapid cooling of electronic components. The cold sprayed coating
may be applied to areas on electronic components that are prone to
overheating. A cold sprayed coating may also serve as a bond layer that
may be low-temperature soldered to a fin structure or a heat exchanger in
an electronic assembly.

[0030]The one or more metals are selected depending on the characteristics
of the polymer substrate, and its intended use. For example, if a bonding
layer is to be cold sprayed between the substrate and an outer layer,
metals that are may be easily adhered to the substrate are selected for
the bonding layer. In addition, one or more hard particles may be
selected to be included to promote wear and erosion resistance. An
exemplary spraying mixture may include a soft metal such as aluminum,
along with hard particles such as aluminum oxide. The selected metals and
any hard particles are combined to form a powder mixture. Mixing the
metal powders may be performed using various hand or machine mixing
methods.

[0031]Next, if the polymeric substrate is a soft material, or if the
selected metal is especially hard, a spraying velocity scheme is designed
as step 32 to avoid damage to the substrate surface. For example, the
spraying velocity may be modified by increasing or reducing the carrier
gas pressure so the sprayed metal particles collide onto the polymeric
substrate without deforming it or otherwise causing component damage.
Determinants such as the type of carrier gas and the particle size for
the sprayed materials may affect the spraying velocity scheme. For
example, although helium is a carrier gas capable of spraying relatively
large particles at supersonic velocities sufficient to cause the large
particles to plastically deform upon impact with a substrate surface,
smaller metal particles can be sprayed at a sufficient velocity using
helium at a lower pressure flow, or using cheaper carrier gases such as
air and nitrogen. Also, the spraying velocity scheme may include
initially spraying with the carrier gas at a low pressure until the metal
forms a thin coating on the substrate. After the thin coating is formed,
the carrier gas pressure may be raised to a relatively high pressure to
form the remainder of the metal coating on the polymer substrate.

[0032]After selecting the one or more metal powders, the system 100 from
FIG. 1 transports the metal with a suitable pressurized gas to the mixing
chamber 26, and the mixture is accelerated by the pressurized carrier gas
through the nozzle 28 toward a targeted surface 10 as step 34. The metal
particles impact with the targeted surface with at least the initial
impacting metals micropenetrating the polymer or composite surface,
meaning that the particles slightly impact the substrate without causing
the overall structure to deform or melt. The sprayed metals bond with the
targeted surface and form a dense coating. Using the above method, a
coating having a substantially uniform microstructure and composition is
bonded to a polymeric substrate. The coating process may be performed
without substantial surface preparation such as grit blasting or chemical
treatments. A clean polymer substrate may be coated with a suitable metal
without any substrate modification or adaptation.

[0033]FIG. 3A is a 30× microscopic image of a cold sprayed aluminum
coating 50 formed on a polycarbonate substrate 52, and FIG. 3B is a
200× microscopic image of the same coating depicted in FIG. 3A. As
seen in the image, only one powder was deposited on the substrate 52
without any bond layer.

[0034]At low magnifications the interface between the coating 50 and the
substrate 52 reveals a discrete boundary. At somewhat higher
magnifications it is apparent that some penetration into the polymer has
occurred. The degree of penetration apparently positively correlates with
the sprayed powder particle size. A clearer image of cold sprayed metal
penetrating a polymer substrate is depicted in FIG. 4, which is a
300× microscopic image of a partial coating of aluminum on a
polycarbonate substrate after just one low velocity coating pass. The
metal powder is penetrating the polymer and the particles are
mechanically held in place. Subsequent passes, which may be performed at
higher velocities, cause the newly-sprayed powder to deform and
metallurgically bond to the powder held in the polymer. The
previously-sprayed powder will also be deformed laterally by the
newly-sprayed powder, forming a thick and uniform coating. Images at even
higher magnifications further reveal the complexities of the mechanical
interlocking between the coating and the polymer, resulting in a strong
bond.

[0035]The kinetic energy associated with the impacting sprayed metal
powder may potentially cause minor substrate and/or powder surface
melting to occur. However, the images in FIGS. 3 to 4 do not reveal any
substantial chemical bonding between the polycarbonate and the sprayed
metals that would be indicative of surface melting. While there is
potential for some chemical bonding between the polymer and the metal to
occur, it is apparent from the images in the figures that the bonding is
substantially mechanical.

[0036]FIG. 4 also reveals larger metal powder particles bond more strongly
to the polymer substrate than do smaller metal powder particles. Thus, an
exemplary cold spraying method includes initially spraying metal powders
having a relatively large particle size, followed by spraying metal
powders having a relatively small particle size. For example, cold
sprayed metal powders may initially have an average particle size of
greater than 50 microns in order to create an initial deep bond zone, and
the subsequently sprayed metal powder may have an average particle size
in the range 5 to 50 microns.

[0037]The cold spraying method of the present invention therefore provides
a strong mechanically-bonded metal coating on polymer components. Such
components may be used in demanding applications because the bond is
capable of withstanding numerous thermal cycles without debonding. High
conductivity metals such as aluminum and copper may therefore be used as
coating metals without having to consider factors such matching the
thermal expansion coefficient of the metal to the polymer, or building up
transition layers to reduce the impact of thermal expansion mismatches.
Similarly, the bond is sufficiently strong for hard wear and erosion
resistant coatings to be deposited without a likelihood for debonding to
occur during use due to mechanical forces on the coating.

[0038]While the invention has been described with reference to a preferred
embodiment, it will be understood by those skilled in the art that
various changes may be made and equivalents may be substituted for
elements thereof without departing from the scope of the invention. In
addition, many modifications may be made to adapt to a particular
situation or material to the teachings of the invention without departing
from the essential scope thereof. Therefore, it is intended that the
invention not be limited to the particular embodiment disclosed as the
best mode contemplated for carrying out this invention, but that the
invention will include all embodiments falling within the scope of the
appended claims.